Stub in more IPC attachment brokering functionality.

ipc/ipc_message.h

Index: ipc/ipc_message.h
diff --git a/ipc/ipc_message.h b/ipc/ipc_message.h
index 9bb7a287c0653743404e116e1bf26988d9458d48..2d1e2ac6c37e9db1b04aa801d82f62ac2a054b9c 100644
--- a/ipc/ipc_message.h
+++ b/ipc/ipc_message.h
@@ -177,6 +177,8 @@ class IPC_EXPORT Message : public base::Pickle {
   bool HasAttachments() const;
   // Returns true if there are any MojoHandleAttachments in this message.
   bool HasMojoHandles() const;
+  // Whether the message has any brokerable attachments.
+  bool HasBrokerableAttachments() const;
 
 #ifdef IPC_MESSAGE_LOG_ENABLED
   // Adds the outgoing time from Time::Now() at the end of the message and sets